Process for chemically forming oxide films on the surfaces of aluminum and aluminum alloys

ABSTRACT

A PROCESS FOR CHEMICALLAY AND NOT ELECTROLYTICALLAY FORMING A THICK, WEATHER-RESISTANT, ANTI-CORROSIVE OXIDE FILM ON THE SURFACES OF ALUMINUM AND ALUMINUM ALLOYS IN A SHORTER PERIOD THAN ELECTROLYTICALLY, WHICH COMPRISES COMPLETING THE PRIMARY REACTION FOR FORMING ALUMINUM HYDROXIDE FILM ON THE SURFACE OF ALUMNIUM OR ALUMINUM ALLOY AND THE SECONDARY REACTION FOR FORCIBLY OXIDIZING AND DEHYDRATING THE ALUMINUM HYDROXIDE FILM TO CONVERT IT INTO ALUMINUM OXIDE FILM SIMULTANEOUSLY IN A SOLUTION.

United States Patent O 3,681,149 PROCESS FOR CHEMICALLY FORMING OXIDE FILMS ON THE SURFACES OF ALUMINUM AND ALUMINUM ALLOYS Hikaru Ito, Green Haitsu, 9540, Katsuragi-rnachi, Kishiwada-shi, Osaka, Japan N Drawing. Filed Feb. 25, 1970, Ser. No. 14,239

Int. Cl. C23f 7/06 US. Cl. 1486.27 Claims ABSTRACT OF THE DISCLOSURE A process for chemically and not electrolytically forming a thick, weather-resistant, anti-corrosive oxide film on the surfaces of aluminum and aluminum alloys in a shorter period than electrolytically, which comprises completing the primary reaction for forming aluminum hydroxide film on the surface of aluminum or aluminum alloy and the secondary reaction for forcibly oxidizing and dehydrating the aluminum hydroxide film to convert it into aluminum oxide film simultaneously in a solution.

The present invention relates to a process for forming a. film chemically in the surface of aluminum or aluminum alloy for preventing corrosion of said aluminum or aluminum alloy.

Heretofore, the following methods have been employed for the formation of a film on the surface of aluminum or aluminum alloys:

(a) Anodic oxidation method (an electrolytic method normally called alumilite process) (b) Chemical film-forming method (generally used for undercoating for painting and, for example, alodine process, M.B.V. process and E.W. process) (c) Coating method (using various types of resins for coating) Of these three methods, method (a) is most excellent in respect of the physical and chemical properties of the film formed, and about 90% of such films are being formed by the method. This is solely because the surface of the substrate aluminum or aluminum alloy is a metal oxide per se.

However, the anodic oxidation method (a) calls for a considerable expenditure for the facility and power consumption for the electrolysis of a material to be treated, as well as large labor for the control of power supply, and consequently the cost of the product becomes high, as is well known.

In order to obviate the shortcomings of the anodic oxidation method described above, the present invention has for its object the provision of a process for chemically economically forming a thick oxide film on the surface of aluminum or aluminum alloys in a relatively short period of time.

According to the present invention, there is provided a process for forming an oxide film on the surface of aluminum or aluminum alloys, which comprises completing the primary reaction for forming aluminum hydroxide on the surface of aluminum or aluminum alloy and the secondary reaction for forcibly oxidizing and dehydrating the aluminum hydroxide to convert it into aluminum oxide, simultaneously in a solution. Such reactions are represented by the following chemical formulae:

Al+3(0H)- Al(OH) (the primary reaction) 2A1(OH) 3H O A1 O (the secondary reaction) Patented Aug. 1, 1972 The aluminum oxide film thus formed is white in color, unlike the transparent conventional film, and has a thickness ranging from 6 to 25 microns, depending upon the composition of the solution and the period of immersion.

If, in the step of forming the aluminum hydroxide described above, a salt of a heavy metal (e.g. iron, copper, nickel, manganese, cobalt or chromium) is added to the treating solution to concurrently form a hydroxide of such heavy metal on top of the aluminum hydroxide film, the resultant oxide film will have a color peculiar to the heavy metal.

The colored compound thus obtained is totally different from the conventional organic or inorganic colorations or pigments present in paints, but is a pure oxide. Therefore, the color is exactly the same as one naturally developed by anodic oxidation (such as Kalcolor of Kaiser Company or Duranodic-300 of Alcore Company) and is obviously highly resistive to weather.

A variety of colors can be obtained, such as gold color, yellow, dark green, reddish black, black (use of 56S and 868 is not particularly necessary), gray (use of 438 is not particularly necessary), brown, orange, olive color and white.

(I) The process comprises the following steps:

Degreasing of a sample-washing with wateretching w th caustic sodawashing with water-neutralization with nitric acid-washing with waterwashing with water-immersion in a treating liquid-washing with water-washing with hot waterdrying-resin coatingbacking.

(II) The treating liquid is broadly divided into a main liquid and a subsidiary liquid.

(a) Main liquid-The main liquid consists of hydrogen peroxide (at a variable concentration) or aqueous ammonia (at a variable concentration) and is used for the sole purpose of forming aluminum hydroxide on the surface of aluminum or aluminum alloy.

(b) Subsidiary liquid.-The subsidiary liquid comprises:

(1) Aluminum hydroxide elution preventing agent and forcibly oxidizing agent (2) Formed film reinforcing agent (3) Salt corresponding to the desired color (IH) Treating time.-Aluminum or aluminum alloy 1s immersed in the treating liquid for a period ranging from 20 to 30 minutes, depending upon the desired film thickness.

(IV) Treating temperature-The treating temperature ranges from 17 to 30 C (V) Water used-Industrial water may be used. (VI) Purity of the chemicals used.Industrial chem- 1cals of commercial grade can be used. The present invention will be further illustrated hereunder by way of example thereof.

EXAMPLE 1 A sample material was immersed in a mixed solution, conslsting of -500 cc. of H 0 400-0 cc. of H 0, 10-40 g./500 cc. of K=F, 0.1-5 g./500 cc. of KBrO 0.55 g./500 cc. of NH F-H'F and 0.1-5 g./500' cc. of C H O at a temperature of 17-25" C. for a period of 20 minutes. A white film of 9-13 microns in thickness was obtained.

EXAMPLE 2 A sample material was immersed in a mixed solution consisting of 50-500 cc. of 3NNH OH, 4500 cc. of

H O, 5-15 g./500 cc. of KP, 1-10 g./500 cc. of NH CI, 1-5 g./500 cc. of KBrO and 1-5 g./500 cc. of CoCl at a temperature of 17-25 C. for a period of 20 minutes. A white film of 7-9 microns in thickness was obtained.

Now, the advantages of the present invention will be set forth below in comparison with the conventional anodic oxidation method. In the below description, character, A represents the conventional method and B the process of this invention.

1) Facility advantage (a) Rectifier:

A: Essential for electrolysis regardless of size. B: Not necessary.

(b) Refrigerator:

A: Necessary for maintaining sulfuric acid electrolyte at 20zt2 C., oxalic acid electrolyte at 301-2" C., and for maintaining the electrolyte at 2011 C. for natural color development.

B: Not necessary. Well water is sufficient to maintain the treating liquid at 17-30 C.

(c) Meters, and negative and positive electrode busbars:

A: Meters are necessary. One of carbon, lead, stainless steel and aluminum is necessary for negative electrode and a copper bus-bar is necessary for positive electrode.

B: Not necessary.

(d) Lead wires, bolts and holders:

A: For conducting current, lead Wires (aluminum), bolts (aluminum and titanium) and holders (aluminum) are necessary.

B: Lead wires and bolts are not necessary. Holders are necessary.

(2) Advantages in case of mass production (a) Liquid temperature control:

A: Difiicult in view of the tolerance of 12 C. and

B: Easy because the liquid temperature may be in the range of 17 to 30 C.

(b) Unsatisfactory contact:

A: Unsatisfactory contact of lead wires and at the bolts due to overheating is possible.

B: Free of unsatisfactory contact and available percentage increases.

(c) Irregularity in film thickness:

. A: The proportion and distance between the negative electrode and the positive electrode largely affect the film thickness and cause a color unevenness in case of natural color development.

B: Less and within 1 micron.

(d) Effect for hollow materials and pipes:

A: A film cannot be formed on the inside wall of such materials unless an auxiliary electrode is used.

B: A film can be formed on the inside wall and a thickness irregularity of the film is about 0.5 micron.

(e) Adaptability to conveyor system:

A: Impossible by reason of current supply. B: Possible.

(f) Productivity:

A: One cycle of operation requires 30 minutes. The productivity is restricted particularly by the capacity of the rectifier used.

B: Even a colored film can be formed in a thickness of 6-7 microns in a period of 20 minutes. The film-forming time is shortened as compared with that of the conventional method and is not restricted by a rectifier at all, so that the productivity is much higher than that of the conventional method.

(g) Manpower:

A: 13 persons for the production of 200 tons/month. B: 5 persons for the production of 400 tons/ month, when a conveyor system is employed.

(h) Chemical cost:

A: 8 Yen/m. for electrolysis in sulfuric acid (including the pre-treatment), l6 Yen/m. for electrolysis in oxalic acid, and 45 Yen/m? for natural color development.

B: 12 Yen/m. for White film and 25 Yen/m. for films of other colors.

(i) Electricity expense:

A: 25 Yen/m. for electrolysis in sulfuric acid, 40 Yen/m. for electrolysis in oxalic acid and 70 Yen/m. for natural color development.

B: Zero.

(1') Quality of material:

A: Subject to limitation particularly in case of natural color development. B: Not subject to limitation even in case of colored films.

(k) Color of film:

A: In natural color development, the colors are limited to gold, amber and black.

B: Any color can be obtained as desired by suitably selecting the type and mixing ratio of heavy metal salt.

(3) Properties of film (a) Composition of film by X-ray difliractiometry:

A: 'YA1203XH20 B: Al O -a minute quantity of H 0 (b) Surface condition observed through electron microscope:

(c) Physical and chemical performances:

The following tests were conducted on the present film in comparison with a conventional one which had been formed by a process comparising the steps of anodic oxidation, washing with water, closing pores, clear painting and baking.

Pencil hardness, cross cut test, impact test, wear proof, chemical resistance (5% caustic soda, 5% hydrochloric acid), salt spray test, weather resistance, mortar resistance, exposure test under irradiation of xenon are lamp (for one year and three months).

The test results revealed that the present film was not inferior to the conventional one.

What is claimed is:

1. The method of non-electrolytically forming an aluminum oxide film on the surface of a metal body consisting at least predominantly of aluminum which comprises treating said surface with an aqueous solution containing an oxidizing agent selected from the group consisting of hydrogen peroxide and ammonium hydroxide together with potassium fluoride, potassium bromate, and an ammonium halide selected from the group consisting of the fluoride and the chloride, thereby forming aluminum hydroxide on said surface, and concurrently dehydrating said aluminum hydroxide to form aluminum oxide.

2. The method of claim 1 in which said solution is at a temperature of from 17 to 30 C.

3. The method of claim 1 in which said solution also contains a sugar having the empirical formula C H O 5 4. The method of claim 1 in which said solution also contains a heavy metal salt to provide a colored coating. 5. The method of claim 4 in which said heavy metal salt is cobalt chloride.

References Cited UNITED STATES PATENTS 1,551,613 9/1925 Pacz ..1486.27

6 FOREIGN PATENTS 823,4571 5/1957 Great Britain 1486.27

RALPH S. KENDALL, Primary Examiner C. WESTON, Assistant Examiner US. Cl. X.R. 106-14 

